Since The M16A2 Product Improvement Program (1980-1983) was my program, this is the down & dirty on the barrel thickness issue.

We (Marines) were replacing a lot of "bent" barrels that were determined to be "bent" because the Armorer's Bore Drop Gauge would not freely pass through some barrels during Ordnance Inspections (LTI's). So the Logisitcs people had "Barrels Bending" on their list of "M16A1" things to "Improve" right after listing "Handguards Breaking."

We "experts" thought this bending was from rough handling like during bayonet drills, etc., as an absence of any mid-barrel handguard damage in these rifles made one assume the fulcrum of such bending was the bayonet lug. So we made that part of the barel thicker because we did not want the excess weight of a full length heavy barrel. In testing using the bayonet lug as a fulcrum, and applying calibrated mechanical pressure to the muzzle, the new barrel was about 9 times more resistant to bend and take a set than an M16A1 profile. So we went with this "improvement."

However, soon after I started using a bore scope with a video recorder and monitor to inspect "bent" barrels. What I found was a mound of bullet jacket material at their gas ports. This build up was caused by a burr left from drilling/reaming the gas port. This was where the Armorer's Drop Gauge was geting stuck. When we removed this "mound", the barrels would all pass the Drop Gauge. We let Colt know what we had deduced, and that is one reason they kept models of "A2's" in their line-up with A1 profile barrels. However, the A2 profile was already down the road for the US Military. So about the only advantage of the A2 profile was to give the rifle a little more muzzle hang. This was noted by most all the Operational Test paticipants, especially when they fired the standing/off-hand leg of our rifle qualification course.

So my advice to mlitary armorers is to never replace a bent barrel until you visually check the gas port, or at least scrub the hell out of the gas port area with a new bore brush and an electric drill. And thank God for chrome bores!